FROM THE BENCH
HYBRID LOG-GAMMA AN ALTERNATIVE HIGH DYNAMIC RANGE (HDR) CONCEPT HAS JUST BEEN RATIFIED BY INTERNATIONAL STANDARDS BODY THE ITU AS PART OF STANDARD BT.2100. IT IS LIKELY TO BE ADOPTED FOR BROADCAST TV. ALAN BENNETT WALKS US THROUGH IT
ybrid Log-Gamma is a relatively simple way of implementing HDR. Developed by the BBC and Japan’s state broadcaster NHK, it’s robust, royalty-free and even backwards-compatible for TVs without advanced HDR hardware built in. Software and firmware (the terms are both used in this application) are held in memory at the receiver. They instruct the central processing unit and other sections on how to conduct themselves, and are used to purge bugs, add new features and benefits and to confer compatibility with new devices, services and broadcast parameters. For any system it’s good to check and update the software to the latest version where the facility is available. The transfer generally takes five to ten minutes, sometimes longer.
A gamma feature has been part of TV broadcasts virtually from the start; it was provided to compensate for the non-linear characteristic of glass picture tubes, whose light output is not proportional to drive voltage. This pre-correction transfer curve restored optimum grey-scale reproduction. Flat-panel screens do not require such correction, but a gamma curve is still given to video signals to best adapt their bit-rate to the capabilities of the human eye. In an HLG system the brighter parts of the picture are given a different (logarithmic) curve shape to provide a much wider range of brightness levels and thus increase the dynamic range of the image (see graph). This concept and its implementation is easily handled by broadcasters, and is not vulnerable to the problems that can arise with the metadata involved with existing HDR systems such as HDR10 (the most established variant) and Dolby Vision, while making less bandwidth/bit-rate demand on the delivery system than the latter. Metadata is an instruction code, broadcast alongside the basic picture data, to ‘steer’ light intensity levels. It is fine for movies and similar content, but does not lend itself to live broadcasting, where conditions change too quickly for it to keep up in terms of light level and TV-internal display control. The use of metadata also involves complex system-specific hardware in the TV, while HLG calls for relatively
extremes of bright sunlight and nearblackout. It could also be applicable to 1080p (HD) broadcasts. HDR provides a great enhancement to image quality, especially on small screens, say below 46 inches, where the increased detail of UHD pictures is difficult to discern unless viewed close up. At present HDR is not available on mainstream TV broadcasts. It is confined to on-line viewing – mainly subscription platforms Amazon and Netflix – and the latest UHD Blu-ray players and discs.
SETMAKERS AND PLATFORMS
2HLG image transfer curves
simpler chips, adaptable with a firmware update to the new system. For more details on HDR, its effect and encoding see my column in the Jan/Feb 2016 issue of Get Connected, available on the website at gcmagazine.co.uk
The beauty of the HLG system is its backward compatibility with standard TVs: they simply ignore the top half of the transfer curve, showing a picture as good as normal, while HLG-enabled models use it to map out and display a much wider range of light levels. Thus the one signal, needing no auxiliary data, can simultaneously convey standard and HDR pictures, obviating the need for simulcasting. The HLG standard also provides for high frame rate operation (up to 100-120 frames per second, good for fast motion reproduction), and picture brightness adaptation to the ambient light level in the viewing area, very different between the
Industry take-up of HDR is ramping up fast at present, and the Dolby system has been gaining acceptance amongst TV manufacturers other than LG, its main supporter up till now. In recent times setmakers and others have shown much interest in the HLG system. JVC’s 2017 range of UHD projectors will incorporate it; also those from Sony, all of whose UHD direct-view models this year will be HLG-enabled. Samsung’s 2016 UHD TVs are HLG-compliant after a firmware tweak; likewise LG’s 2016 OLED and SUHD models. The latest crop of OLED screens from LG, Panasonic and Sony also support HLG, in some cases requiring a software update. Regarding connections, the current HDM2.0b coupler is HLG-friendly, while versions 2.0 and 2.0a will probably work in most cases. Amongst broadcasters and platforms, the BBC, Digital UK, Eutelsat, Freeview Play and Sky intend to use HLG. Google’s Android TV v7.0 embraces HLG, also to be taken up by YouTube. Quite a list! The new TV models introduced at CES in January gave a boost to HDR and HLG. Notable amongst them is Sony’s new OLED product with Dolby Vision, and the newly launched Samsung QLED range; and particularly Panasonic’s 65-inch type EZ1002, with a new colour picture processor chip HCX2 having studio-derived 3D look-up tables for consummate image transfer and mapping. In terms of picture dynamic range this model has the virtue of a very bright LED display, ranging up to 800 nits, twice that of the company’s first such offering. For people like me who generally watch movies in subdued light - as they were intended to be seen - this peak brightness level, combined with a ‘total-black’ capability, seems to be the optimum display for HDR pictures, however they are conveyed.
HDR is advancing rapidly, and amongst the competing systems Hybrid Log-Gamma is gaining ground very quickly. While it may prove not to be able to provide better images than its rivals, its simplicity, ruggedness and compatibility may well win for it the race for acceptance, at least amongst broadcasters and setmakers. MARCH 2017 GET CONNECTED
Published on Mar 20, 2017
Published on Mar 20, 2017
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